1. Field of the Invention
This invention, in its broad aspect, relates to neutron, hard X-ray, and gamma ray imaging, and to Fourier imaging instruments therefore. In a more specific aspect, the invention relates to high precision grids or arrays for such imaging instruments. In still another aspect the invention provides a method for the fabrication of such grids.
2. Background Information
During the past three decades, observational astronomy has expanded from the relatively narrow wavelength band of visible light to the entire electromagnetic spectrum. In addition, subatomic particles especially high-energy neutrons form a second spectrum of interest. The impetus for this expansion was the realization that different spectral ranges allow different and complementary insights into cosmic and other natural events as well as enabling images of objects and masses surrounded by ordinarily opaque materials.
X-ray astronomy is a product of the Space Age, enabling observers to cover a band of photon energies between 0.1 keV and 500 keV. Gamma rays have even higher photon energies. The X-ray sky is dominated by active sources such as radio galaxies, Seyfert galaxies, and quasars, which emit X-rays and gamma rays, as well as black holes, and clusters of galaxies that make up the largest physical formations of our universe.
Significantly, phenomena that occur at the end of the stellar lifetimes are observable in our Galaxy and beyond. Such stars as White Dwarf stars, neutron stars and pulsars emit neutrons that can be also be studied. In addition to sources located in the heavens, many terrestrial applications also employ the penetrating characteristics of x-rays, gamma rays, and neutrons. The invention herein is therefore concerned with neutron, hard X-ray, and gamma ray imaging.
Hard X rays, gamma rays, and high energy neutrons cannot be reflected or focused with lenses or mirrors. Impinging at normal incidence on optical materials, they penetrate the optic rather than experiencing the refraction (lenses) or reflection (mirrors) necessary to form an image. Hard X-ray astronomy (20 to 100 keV) and other imaging applications were originally handicapped because of this lack of imaging capability. Further, it was realized that even grazing-incidence reflection, used very effectively in soft X-ray astronomy, is impractical in the photon-energy domain above a few keV. This realization led to the development of Fourier telescopes, one such telescope being the subject of U.S. Pat. No. 5,838,757. Fourier telescopes permit observations over a very broad band of energy from ultraviolet to 100 keV.
Soft X-ray telescopes using multilayers are based on designs that utilize arrays of crystals that are adjusted to diffract photons of a fixed energy to the same point along the optical axis. Crystals have been used to diffract X-rays for years. Their periodic structure makes this possible. Crystal diffraction gratings, however, are not the perfect solution to the X-ray astronomy problem. As pointed out in U.S. Pat. No. 4,675,889 crystalline structures such as lithium fluoride, metal acid phthalates, and pyrolytic graphite have very restrictive lattice spacing constraints, and they must be operated near room temperature in a dry environment. It is noted in U.S. Pat No. 5,646,976 that crystals also possess poor mechanical qualities such as resistance to scratching. For such reasons, as expressed in U.S. Pat. No. 4,675,889, numerous steps have been taken to construct both natural and new crystalline analogue materials. Such attempts have led to synthetic structures, known as multilayers or multilayer coatings, consisting of alternate layers of high and low atomic number elements, termed high-Z and low-Z materials. Multilayer coatings are the subject of such patents as U.S. Pat. No. 4,675,889, U.S. Pat. No. 4,915,463, U.S. Pat. No. 5,042,059, U.S. Pat. No. 5,646,976, U.S. Pat. No. 5,757,882, and U.S. Pat. No. 5,799,056. Referring again to U.S. Pat. No. 5,646,976, in order for a multilayer structure to reflect by imitating a crystal structure, a light element of the lowest possible electron density is layered with a heavy element of the highest possible electron density. This means that fabrication of the multilayer structure is not without its difficulties. The angle of incidence and d spacing must be manipulated according to the Bragg equation.
The coating of optical surfaces without imperfections in d spacing is difficult. In an effort to obviate such difficulties multilayers have been formed by electron beam-physical vapor deposition, laser evaporation, sputtering techniques such as magnetron RF, ion beam and bias sputtering, as well as diode sputtering, reactive gas injection and the standard multisource evaporation process disclosed in U.S. Pat. No. 4,675,889. These methods, while solving the problem, have been costly and time consuming. They have rendered the multilayers one of the more expensive elements of an imaging system.
Many of these same methods have been attempted for hard x-ray, gamma ray, and neutron imaging systems with the same prohibitive costs being realized. This and other disadvantages have led to other approaches such as milling and etching, and the placement of thousands of tiny slats into pre-machined slots in telescope grid trays. Fabricators have even attempted to place each single slat, or layer, in place by hand in a frame in lay-up fashion Typically such methods have been very time consuming, and they have been disadvantageous from a performance standpoint. By the practice of this invention the difficulties and expense of deposition coatings to produce multilayer grids are overcome, as well as problems encountered in the undesirable slat hand-fabrication method.
An object of the invention is to provide a low cost approach for fabricating grids, while retaining or improving performance.
Another object is the provision of grids and a method of fabricating grids for use in Fourier Imaging Systems in any configuration required by the imaging system.
Still another object of the invention is the provision of a method that greatly reduces grid fabrication times.
Imaging information has frequently been collected using multiple grids mounted in grid trays within telescopes and other optical instruments used for neutron, hard X-ray and gamma-ray imaging. By this invention multilayer grids are provided for telescopes carrying grid trays having openings to receive such multilayer grids. A polyhedron is fabricated with two larger faces in the form of congruent polygons that form front and back surfaces so sized that the resulting polyhedron fits slidably within a grid tray opening. Smaller polygonal faces separate the front and back surfaces, forming a polyhedral grid case. All of the polyhedral faces are transparent to photons of interest. For x-rays and gamma rays, alternate layers of a high-Z material and a low-Z material are then inserted in the polyhedron, through an open face of the polyhedron. The layers of high-Z and low-Z materials are so sized that their widths are equal to the width of the polyhedron between the front and back faces. The inserted layers are then uniformly compressed to form a multilayer grid. Desirably the compressing operation is accomplished by the use of one or more pistons. For neutron imaging, alternating layers of beryllium and glass may be used respectively. In all cases, the idea is to alternate materials that are opaque/absorptive and transparent to the selected photon or particle.
This invention provides a grid system that normally includes grids arranged in a grid array that can be rotated to produce components in a Fourier transform to synthesize an image of an object being viewed. The imaging methods are applicable to various energies of penetrating radiation, and they are particularly suitable for neutrons, hard X-rays, and gamma rays for which there are no other effective imaging methods. With the understanding that the thicknesses of the multilayers are overemphasized for the purpose of clarification the invention it will now be described in conjunction with drawings of some of its embodiments as well as of a preferred embodiment.